1. Field of the Invention
The present invention relates to an exposure apparatus and an image forming apparatus using the same.
2. Description of the Related Art
An electrophotographic image forming apparatus generally exposes the surface of a photosensitive member to a light beam (laser beam) output from a light source (laser light source) provided in an exposure apparatus such as an optical scanning apparatus, thereby forming an electrostatic latent image on the surface of the photosensitive member. The photosensitive member on which an electrostatic latent image is formed may have a sensitivity which varies in each individual position (region), that is, unevenness of sensitivity on its surface. This unevenness of sensitivity may occur due to the difficulty in maintaining the thickness of a photosensitive layer of a photosensitive member constant in a process of manufacturing the photosensitive member. When the photosensitive member has unevenness of sensitivity, the surface potential of the photosensitive member varies in each individual region even if the surface of the photosensitive member is charged by a charging device under the same conditions and exposed by an exposure apparatus under the same conditions. As a result, potential unevenness (error) may occur in this surface potential. Therefore, if unevenness occurs in the potential characteristics of the surface of the photosensitive member, density unevenness may occur in the formed image upon development of the electrostatic latent image using a developing material such as toner.
As a technique to compensate for such an error of the surface potential of the photosensitive member due to its unevenness of sensitivity, a technique as described in, for example, Japanese Patent Laid-Open No. 2004-223716 has been proposed. According to this patent literature, in an image forming apparatus including a single laser light source, the power of light from the laser light source is controlled using a correction value corresponding to unevenness of sensitivity of a photosensitive member so as to uniform the surface potential (each of the dark portion potential and light portion potential) of an electrostatic latent image formed on the photosensitive member. More specifically, a correction value for the sensitivity in each region on the photosensitive member is determined from sensitivity data representing this sensitivity, and a drive current to be applied to the laser light source is corrected in accordance with the determined correction value.
In light power control described in Japanese Patent Laid-Open No. 2004-223716, a drive current applied to the laser light source is corrected in accordance with unevenness of sensitivity of the photosensitive member to correct, in turn, an error of the surface potential of the photosensitive member. In forming an image, the laser light source irradiates the photosensitive member with a laser beam in light power corresponding to image information in accordance with light emitting characteristics determined depending on the drive current controlled by such light power control. However, when the drive current applied to the laser light source is changed by such light power control, the light emitting response of the laser light source changes. In this case, due to this change in light emitting response, the light emitting characteristics of the laser light source shift from ideal light emitting characteristics determined depending on the drive current. Especially when the drive current is modulated (by, for example, pulse width modulation) in accordance with image information so that the laser light source emits light in power corresponding to the image information, an error may occur between the actual power of light and the ideal power of light determined depending on the drive current. As a result, when the photosensitive member is irradiated with a laser beam corresponding to the image information, the above-mentioned error resulting from unevenness of sensitivity of the photosensitive member remains in the surface potential of the photosensitive member, so the image quality may degrade.